1. Field of the Disclosure
The present disclosure relates to an optical waveguide and an arrayed waveguide grating, which can reduce insertion loss when light enters from a slab waveguide toward an arrayed waveguide or when the light enters from the arrayed waveguide toward the slab waveguide.
2. Description of Related Art
In a DWDM (Dense Wavelength Division Multiplexing) multiplexer/demultiplexer, an M×N star coupler, a 1×N splitter, and so on, Patent Documents 1 to 6 disclose such a connection structure between a slab waveguide and an arrayed waveguide that when light enters from a slab waveguide toward an arrayed waveguide, the light does not radiate in a clad layer as a radiation mode between the arrayed waveguides adjacent to each other.
In the Patent Documents 1 to 4, a transition region where the refractive index of the waveguide gradually changes from the slab waveguide toward the arrayed waveguide is disposed. In the Patent Document 5, a slope portion is disposed between the slab waveguide and the arrayed waveguide. In the Patent Document 6, a core layer and a plurality of island-shaped regions are arranged in the slab waveguide. The refractive index of the island-shaped region is smaller than the refractive index of the core layer. The island-shaped regions face a clad layer provided between the adjacent arrayed waveguides. The width of the island-shaped region in a direction substantially vertical to a light propagation direction becomes narrower from the slab waveguide toward the arrayed waveguide. Light passing through the core layer provided between the island-shaped regions adjacent to each other propagates toward the arrayed waveguide without changing the propagation direction. Light passing through the island-shaped region changes the propagation direction due to a tapered shape of the island-shaped region and propagates toward the arrayed waveguide. The tapered shape and the position of the island-shaped region are optimized, whereby the light is concentrated on the arrayed waveguide and propagates in the arrayed waveguide as a propagation mode.